Ah, here's where I left off, found it ...
Picking up from
based on the article at : https://web.archive.org/web/20170909062232/http://alistair.cockburn.us/get/3598
(15) Effectiveness per person drops as people are
added to a project.
Number 15 is a straightforward consequence of
the communications and coordination load in-
creasing as people are added to a project.
@totheralistair I call shenanigans on this. I have a blog post way down my list somewhere on what I'm calling the Combinatorial Fallacy, but the tl;dr is that n people don't have n! conversations unless you are doing it spectacularly wrong.